Medical ultrasound transducer having non-ideal focal region

ABSTRACT

A mechanically formed transducer capable of producing a non-ideal focal region is described. The transducer has a plurality of piezoelectric elements suspended in an epoxy and heat molded into a desired shape. One or more shaped irregularities in the transducer provides for a mechanically induced non-ideal focal field without the need for electronic steering or lens focusing. Systems and methods of making the same are also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation-in-part from U.S. patentapplication Ser. No. 10/816,197 entitled “Vortex Transducer,” (AttorneyDocket No. 021356-000320US), filed on Mar. 31, 2004, which claimed thebenefit of Provisional Application No. 60/459,355 (Attorney Docket No.021356-000300US), filed Mar. 31, 2004, and of Provisional ApplicationNo. 60/483,317 (Attorney Docket No. 021356-000310US), filed on Jun. 26,2004, the full disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultrasound transducer for the use inhigh intensity focused ultrasound (HIFU) applications for medicalapplications.

2. Description of the Background Art

Lysis (the process of disintegration or dissolution) of human tissueusing high intensity focused ultrasound (HIFU) is a technique that hasbeen studied for over 50 years. Research into applications of HIFU haverevolved mostly around treating malignant tumors in the body eitheruntreatable by other means, or promising a more efficacious treatmentmodality. HIFU commercialization has been very slow to develop, however,despite some of its early promise. Reasons for this include theinability to visualize the lesions being formed, the necessity of havingto lyse an entire malignant tumor to be considered effective, andespecially the extended period of time required to lyse a significantvolume of tissue. In the last 10 years or so, some companies have beenformed to commercialize HIFU for non-cancerous treatment applications.The best-known example is the treatment of enlarged prostate or BPH.Here, it is not necessary to lyse all the tissue to be effective.Advances in the treatment of BPH with drugs, however, has seriouslyshrunk the commercial prospects for this case. Advances in MRI anddiagnostic ultrasound in the last 20 years have aided visualization ofHIFU lesions, alleviating a main obstacle to the commercial advancementof the field. Additionally, where small volumes of tissue can betreated, as in hemostasis and blood clot breakup, HIFU is likely toprove to be viable commercially. Lysing large volumes of tissue, as inthe case of removing significant amounts of adipose tissue, requiresadditional technical strategies.

Cross-sectional histological views of a HIFU lesion formed in vivo inporcine adipose tissue are shown at 40× (FIG. 1) and at 200× (FIG. 2).Tri-chrome staining is employed. Lysed tissue shows blood perfusionaround individual fat cells and incursion of phagocytes and red bloodcells into the HIFU-treated volume. Lesions formed with HIFU aretypically cigar-shaped, with lesions lengths being 5-8 times the lesiondiameters. HIFU lesions are typically formed by spherically focusing anultrasonic beam. These lesions can be as little as 1-2 mm in diameter,and 6-10 mm in length. It would take many lesions to necrose a largevolume of tissue. Preferred thermal processes of gradual heating aregenerally slow, so that it may take up to 30 seconds to generate enoughabsorption of the ultrasonic energy to raise the temperature high enoughto necrose tissue, and additional time to allow the temperature oftissue between the skin and treated volume to cool sufficiently beforethe next lesion is created. A simple calculation shows that it couldtake hours to ablate a volume of tissue on the order of 250 cc with asingle transducer making individual lesions. Diminishing the timerequired to ablate a large volume of tissue, as in adipose tissuereduction, could mean the difference between a successful commercialproduct and an unsuccessful one.

Several strategies can be employed to reduce the time between makingindividual lesions include using multiple mechanically scanned HIFUtransducers, scanning the transducer(s) continuously, and/or using somekind of linear or 2D transducer array or structure to generate multiplefocal spots. While some combination of these strategies could beemployed, a major physical limitation of reducing scanning time remainsthe small diameter of the focused ultrasound beam. A defocusing strategyholds some promise of increasing the effective spot size by spreadingthe energy more laterally than lengthwise, creating a morespherical-shaped lesion. “Wobbling” the HIFU transducer mechanicallyabout its axis could serve to do this, and such ideas have been reportedin the literature. However, it would be certainly less complicated andexpensive to build the defocusing into the HIFU transducer itself.

A method of creating an annular focal zone where the diameter of theannulus is adjustable has been reported by Cain and Umemura (Cain,Charles A. and Shin-Ichiro Umemura, “Concentric-Ring and Sector-VortexPhased-Array Applicators for Ultrasound Hyperthermia”, IEEE Trans. onMicrowave Theory and Techniques, Vol MTT-34, No. 5, May 1986, pp.542-551.) and (Umemura, Shin-Ichiro and C. A. Cain, “The Sector-VortexPhased Array: Acoustic Field Synthesis for Hyperthermia”, IEEE Trans. onUltrasonics, Ferroelectrics, and Frequency Control, Vo. 36, No. 2, March1989, pp. 249-257.) and Hynynen et. al. (Fjield, T., V. Sorrentino, H.Cline, and K. Hynynen, “Design and experimental verification of thinacoustic lenses for the coagulation of large tissue volumes”, Phys. Med.Biol., Vol 42, 1997, pp. 2341-2354.) and (Fjield, T. and K. Hynynen,“Experimental Verification of the Sectored Annular Phased Array for MRIGuided Ultrasound Surgery”, Proc 1996 IEEE Ultrasonics Symposium, pp.1273-1276.) and (Fjield, T. and K. Hynynen, “The CombinedConcentric-Ring and Sector-Vortex Array for MRI Guided UltrasoundSurgery”, IEEE Trans. on Ultrasonics, Ferroelectrics, and FrequencyControl, Vo. 44, No. 5, September 1997, pp. 1157-1167.) and (Fjield, T.,N. McDannold, C. Silcox, and K. Hynynen, “In Vivo Verification of theAcoustic Model Used to Predict Temperature Elevations for MRI GuidedUltrasound Surgery”, Proc 1998 IEEE Ultrasonics Symposium, pp.1415-1418.). This concept, dubbed the sector-vortex array, has beenimplemented using electronic array techniques or simply adding amechanical sector-vortex lens onto the front of a planar transducer. Theeffect of this lens is to create a double-cone field pattern that yieldsan annular ring in cross-section.

Extensive research with mechanically scanned, spherically focusedtransducers has been conducted in HIFU fat-tissue mimicking gel phantomsand in vivo porcine adipose tissue. The purpose of this research is todetermine basic design parameters for optimizing the lysing ofsubcutaneous adipose tissue and develop candidate lysing and scanningprotocols for inclusion into a product development specification andinitial human safety trials. This research has shown that challengesremain in reducing the overall treatment time to the desired values.While various scanning strategies have proven fruitful in reducing theheat build-up tissue in the zone between the skin and HIFU-created focalvolume, and thus reducing scanning time, even greater efficacy can beshown if the focal zone overall diameter is increased significantly.Transducers with built-in non-ideal focal region capability could proveto be a relatively simple, cost-effective means to achieve thiscapability.

Spherical focusing is typically achieved by bonding a plano-concave lenson to the front of a planar piezoelectric material, or shaping thepiezoelectric material into a spherical bowl. High ultrasonicintensities (1-4 kW/cm²) are created driving the piezoelectric materialat very high power levels, and focusing the beam very tightly.Absorption of this energy by tissue elevates its temperature. Raisingthis temperature in excess of 60° C. in the focal zone coagulates cellproteins, thereby ablating the tissue. There is a very sharp linebetween ablated and unablated tissue, as seen in FIG. 1. If theintensity is high enough, the temperature rise is sufficient to causeboiling and the production of small bubbles, producing a lesion with theelongated shape of a weather balloon at high altitudes (“tadpole” shapeto others). At an even higher threshold, inertial cavitation (bubblecreation) may occur, which can lyse tissue through thermo-mechanicalmeans.

Lesions made in vivo in porcine tissue with spherically focusedtransducers show remarkably similar characteristics to those made inadipose tissue. Cigar-shaped lesions are made at relatively low powerlevels at longer insonification times, following the 6 dB contour of aclassically spherically focused lens, and elongated weatherballoon-shaped lesions are made at relatively high power levels wheresuspected boiling creates bubbles which reflect acoustic power back tothe skin surface. At very high power levels, or too long insonificationtimes, excessive heat can be generated above the focal zone which willlyse tissue up to and including the skin surface. While large volumes oftissue can be lysed in this manner, controlling this type of lesiongrowth can be problematic when considering patient and environmentalvariability.

Reducing the focal intensity at the center of the lesion and spreadingthe energy out over a larger volume should help alleviate the creationof a boiling hot spot while allowing more energy to be deposited duringan insonification period, ultimately reducing the scanning time.

Composite piezoelectric materials have been a subject of research anddevelopment for nearly 25 years. Ultrasonic transducers and arrays fordiagnostic imaging purposes have been manufactured since the late1980's, and have application in such diverse areas as sonar andnon-destructive testing. One manufacturer (Imasonic, Besancon, FR)builds custom composite HIFU applicators which are used by severalresearch and commercial organizations. Popularly known aspiezocomposites, these materials are made by taking solid blocks ofpiezoelectric ceramic, dicing the block into a forest of tall, thinpillars of ceramic, and backfilling the dicing kerfs with a polymer.This composite ceramic/polymer material is then processed like a normalsolid transducer ceramic into thin plates. The volume fraction of thecomposite is controlled by the dicing kerf width and spacing, and thecomposite material properties can thereby be tailored to specificapplications.

While many superior properties of composites are exploited in diagnosticimaging, the chief interest for HIFU is the ability to form compositesinto arbitrary shapes. In diagnostic imaging, the superior piezoelectricand acoustic properties of composites are exploited to produce widebandfrequency responses, which are of little interest in HIFU at this time.Flexible polymer materials are often used as composite fillers whichallow the material to be easily manipulated into cylindrical orspherical shapes. However, flexible composites are inherently high lossmaterials and unsuitable to the high power levels used in HIFU. Flexiblematerials would distort and ultimately disintegrate due to the heatbuild-up inside the material itself under high drive levels. Using lessflexible polymers as composite filler materials would increase thecomposites' power handling capabilities due to lower intrinsic lossmechanisms, but would then make forming the composite into a curvedshape seemingly impossible.

The answer to this dilemma is to make use of an interesting property ofmany hardset epoxies: these materials can be heated at a relatively lowtemperature into a partially cured state (namely, a B-stage cure) thatis quite hard, but fairly brittle. In this state, the material can beprocessed, that is diced, filled, ground, lapped, and electroded intothe thin sheets needed for HIFU applicators, and then reheated to atemperature somewhat above the original B-stage cure temperature. Atthis point, the polymer softens considerably and can be clamped into amold shaped to the configuration desired (for instance, a sphericallyshaped bowl or a shape to provide a non-ideal focal region) and reheatedto a much higher temperature. The epoxy filler then will fully cure, andfurther heat treatment can elevate the epoxy glass transitiontemperature, the temperature at which the polymer suddenly will soften,to levels between 120° and 200° C. The material thus formed isrelatively low loss and capable of handling HIFU power levels, albeitwith less efficiency than solid, high power, piezoelectric ceramics.

While requiring custom molds and clamping equipment, ultimately it iseasier and cheaper to produce ceramic elements this way than to grindthem directly using expensive equipment or hand labor; the elements aremore rugged as well, which is a very important consideration. The lowerefficiency is not expected to be a limitation in fat lysis since higherfrequencies can be used for the small treatment depths, and thus highintensities can be achieved with relatively low drive levels. This iseasily demonstrated by considering the fact that the intensity antennagain for a focused radiator increases as the square of the frequency. Anoptimum frequency for HIFU at a given depth of focus can be obtainedfrom the following equation:f_(opt)=1/(2αz),where f_(opt) is the optimum frequency, α is the ultrasonic absorptionof fat, and z is the tissue depth to be treated. For fat treated at 2 cmof depth, the optimum frequency for maximum heat deposition is 4.2 MHz.Other considerations may change the actual operating frequency, ofcourse.

Clearly the marriage of a technique of increasing the focal area of aHIFU applicator like the vortex or non-ideal focal region concept and ameans of easily and cheaply implementing this interesting structure likepiezocomposites can reduce the overall treatment time for lysing largevolumes of adipose tissue. Reducing treatment time and cost are keyareas which will determine the fate of lysis commercialization efforts.

Sector-vortex array design has been extensively described and simulatedby Cain and Umemura, Umemura and Cain, and numerous papers from Hynynenand co-workers at Brigham's and Women's Hospital at Harvard. Theydescribe an implementation whereby the driving electrode of a sphericalradiator is divided into N number of equal sized, pie-shaped sectors.Following Cain, driving signals are applied to the N sectors with aphase distribution over the N sectors determined by:φ=m(θ₁),for l=1,2, . . . N where m is the vortex mode number, and θ₁=i2π/N. Thephase distribution over the N sectors is such that that the excitationfield rotates around the radiator at a phase velocity ω₀/m. When N islarge, an approximate analytic expression for the acoustic field in thefocal plane can be derived; this field has a shape determined by the mthorder Bessel function. The vortex-shaped field is zero along the centralaxis for ≠0, and has a diameter proportional to the vortex mode m. Thefield in cross-section resembles an annulus with side lobes at radiigreater than the annulus.

However sector-vortex designs require complex electronics to drive thetransducers in order to produce the vortex focal field. The electronicsare required to drive either sector transducers or phased arrays insequence. Some prior teachings rely on ancillary technologies such asthe use of MRI machines to detect hotspots, and provide for additionalelectronic to provide real time corrections in the electronic firing ofthe various transducer elements to eliminate or reduce the occurrence ofout-of-field ultrasound excitation. Complex lenses have also beendescribed to facilitate the steering of the ultrasound energy into thebody.

Thus there remains a need in the art for a robust vortex transducer,having a simplified design that can operate without the requirement ofcomplex and expensive electronics.

There is further a need for a vortex transducer that can be aimedwithout the use of a lens.

There is still further a need for a vortex transducer having a fastactivation and treatment time for reliably depositing a fixed amount ofenergy into a focal zone.

BRIEF SUMMARY OF THE INVENTION

At least some of the needs of the art are addressed by the presentinventions. In accordance with the needs of the art, it is an objectiveof the present invention to provide a transducer capable of producing avortex focal zone.

It is further an objective to provide a transducer able to create avortex focal zone without the use of complex electronics.

It is still further an objective of the present invention to be able toreliably aim a vortex transducer without using a lens or electronicsteering.

Yet another objective is to provide for a vortex transducer with a cheapand cost effective manufacturing process.

At least one of these objectives is met with a medical ultrasoundtransducer having an axis with a front and back surface transverse tothe axis. The transducer has an edge or perimeter that is axially offsetto produce at least one substantially annular 2 πm phase shift focalregion(s) when the transducer is excited. The m variable is not a wholenumber. If the transducer has more than one offset, each offset may bethe same or a different value.

In a second embodiment there is a medical ultrasound transducer havingan axis with a front and back surface transverse to the axis. Thetransducer has two or more edges of the surface that are axially offsetto produce a 2 πm phase shift focal region for each offset when saidtransducer is excited.

A method of manufacturing is disclosed comprising the steps of firstshaping a piezoelectric ceramic into a desired form, the form having afront end and a back end. Second dicing the front end to create aplurality of elements, the elements being attached to the back end andseparated by dicing channels. Third, filling the dicing channels with apolymer material and allowing the polymer to gel. Fourth, creating atransducer form by removing the back end such that the elements areseparated from one another. Fifth, pressing the transducer form into amold and heating the transducer form such that the resin is heated abovethe B-stage and allowing the resin to cross-link and cool in a setshape. Sixth, treating at least one surface of the transducer form witha conductive material such that all elements are in contact with theconductive material. An additional step involves the formation of ashaped irregularity to produce at least one substantially annularnon-deal focal zone when the transducer is excited. This step can beinserted into the steps above where ever desired and there is no bestorder for the introduction of this step. The shaped irregularity can bepart of the original forming of the transducer form, or it can becreated later by cutting or grinding the transducer form such that thedesired irregularity is introduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a histological view of a HIFU lesion in porcine adipose tissueat 40× magnification.

FIG. 2 is a histological view of a HIFU lesion in porcine adipose tissueat 200× magnification.

FIG. 3 a is a schematic illustration of a vortex ultrasoundtransducer/applicator design in accordance with the present invention.

FIGS. 3 b and 3 c illustrate the transducer having a 2 πm phase shiftwhere multiple variable values of m are formed.

FIGS. 4 a-4 d show a cross-sectional focal zone intensity distributionsfor a series of sector-vortex applicators, modes 0 through 3,demonstrating the annular structure of the field patterns and thewidening of the annulus as a function of mode number.

FIGS. 5 a and 5 b is a further illustration of an embodiment of thepresent invention.

FIGS. 6 a-6 g illustrate a process for making transducers in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Described herein is an implementation of a version of the sector-vortexconcept. While this concept is known, this disclosure implements thedesign in a novel manner. In order to avoid the complexities and cost ofan electronic beamformer, or the use of a sector-vortex lens with itsattendant issues in reliability, acoustic losses and reverberations, andcooling, a continuous vortex lens can be incorporated into thepiezoelectric transducer material itself. This can be done by buildingthe transducer out of piezoelectric composite material which can bemolded into a vortex lens. This eliminates the need for complex drivingelectronics and all the issues of an external lens. This design could beinstrumental in building cheap, reliable devices that can potentiallyreduce treatment time by factors of two to eight.

The present invention implements an applicator design that incorporatesthe phase shifts mechanically, not electronically. This ismathematically equivalent to letting N→∞, that is letting the number ofsectors become infinite, or making a continuously curving shape, asshown in FIG. 3 a, that yields a 2 πm phase shift after one revolution.The applicator would also be preset with spherical focus. As one cansee, this shape would be difficult to machine in a solid piezoelectricceramic material. But one can easily imagine molding a piezocompositeinto this shape using steel molds manufactured with a ball end mill orEDM (electric discharge machining) machine. FIG. 3 a also illustratesthe generic shape of a continuously variable sector-vortex applicatormade by splitting a spherical bowl along a radius and translating onesplit edge axially in relationship to the other edge.

Alternatively, the transducer face may have two or more offsets in theedge. In FIGS. 3 b and 3 c there is a transducer illustrated as havingthree axial offsets along the transducer edge, and each offset has adifferent value m (m₁, m₂, m₃). While three offsets are illustrated, thenumber of offsets, shape and size of the offsets may be adapted toproduce a non-ideal focal region of any shape and intensity within thelimit of the transducer's total capacity.

A 2 π phase shift (mode number=1) corresponds to one wavelength of thedriving frequency; at 4 MHz in water, this is 380 μm. Thus, for mode 1,the discontinuity at the split on the edge of the spherical bowl wouldthen be 380 μm, mode 2 (being 2 πm as described in the parentapplication), 760 μm, and so on. The mode number may be a non-integral,or non whole number to produce additional variations. For example, ifthe mode number is 1.5, the value of the 4 MHz transducer will be 570μm. Splitting a spherical bowl made of piezocomposite along one radialline, and forming it with a discontinuity on the order of 1 mm ispossible.

FIGS. 4 a-4 d show the cross-sectional focal patterns for modes 0, 1, 2,and 3, respectively. Mode 0 reduces just to the simple sphericalradiator, and serves as the standard by which to judge the increase infocal zone area in other designs.

Theoretically, the field along the central axis of the sector-vortexapplicator is zero and would not contribute to tissue lysis. However,heat generated in the annulus will flow into this region and will nothave ready access to a heat sink. It is expected, if the annulus is nottoo large, that the temperature will rise to therapeutic levels underinteresting drive conditions, forming a large solid lesion.

Based on the proceeding description, it is possible to produce a vortextransducer. The present invention comprises a mechanically formed vortexultrasound transducer capable of producing at least one, substantiallyannular focal region(s) when said transducer is excited. The transducermay incorporate a solid piezoelectric material or a compositepiezoelectric material. Furthermore the transducer may incorporate oneor more matching layers. The transducer may incorporate a fillermaterial in front of the transducer or backing material in back of thetransducer. The transducer may also be formed from a single contiguouspiezoelectric element. See FIGS. 5 a and 5 b, where the referencenumbers are as follows:

-   -   1. Back electrode connection and wire    -   2. Electrical connector    -   3. Transducer housing    -   4. Backing material    -   5. Piezoelectric element    -   5 a. Matching layer(s) on front of piezoelectric element    -   6 a.-6 b. Fluid inlet and outlet for cooling    -   7. Front electrode connection and wire    -   8. Filler material    -   9. Face material    -   10. Piezoelectric material    -   A method of making the transducer of the present invention is        also provided. The method comprises the steps of: first shaping        a piezoelectric ceramic into a desired form, the form having a        front end and a back end. Second dicing the front end create a        plurality of elements, the elements being attached to the back        end and separated by dicing channels. Third filling the dicing        channels with a resin material and allowing the resin to gel.        Fourth, creating a transducer form by removing the back end such        that the elements are separated from one another. Fifth,        pressing the transducer form into a mold and heating the        transducer form such that the resin is heated above the B-state        and allowing the resin to cross link and cool in a set shape.        Sixth, treating at least one surface of the transducer form with        a conductive material such that all elements are in contact with        the conductive material. An additional step involves the        formation of one or more shaped irregularities to produce a        non-ideal focal zone when the transducer is excited. This step        can be inserted into the steps above where ever desired and        there is no best order for the introduction of this step. The        shaped irregularity can be part of the original forming of the        transducer form, or it can be created later by cutting or        grinding the transducer form such that the desired irregularity        is introduced.

The manner of producing the transducer is detailed below. First, thepiezoelectric material is formed into a desired form (FIG. 6 a). Theform may include the irregularity used to mechanically create the vortextransducer effect, or the irregularity can be introduced into thefinished transducer form. The shaping of the piezoelectric ceramic intoa desired form follows procedures well established in the art of makingultrasound transducers. The principal embodiment calls for a flat shapehaving sufficient size to be later molded into the desired threedimensional form. Otherwise there is no restriction or requirement onthe formation process. Similarly the dicing of the ceramic is also wellknown. In the second step the ceramic is diced such that there issufficient depth in the dicing channels to allow a resin like materialto fill the channels (FIG. 6 b). The filling of the channels with apolymer (e.g. epoxy or urethane) provides a solid material to supportand maintain the elements in a fixed position relative to one another(FIG. 6 c). Once the polymer is set sufficiently to hold the elements inplace without the structural support of the ceramic back plate, the backplate is removed (FIG. 6 d). Now the transducer form comprises theindividual elements and the polymer used to suspend the elements andhold them in place.

The resulting two-phase composite material can be tailored to have anumber of interesting electro-mechanical and purely mechanicalproperties by the choice of parameters like ceramic volume fraction andpolymer filler material. Of particular interest in permanently formingpiezocomposities into interesting shapes is the use of hardset (highdurometer) thermoset resin systems such as epoxy as the polymer filler.The set epoxy in the composite can be partially cured at a relativelylow temperature where it “gels,” or turns substantially solid from aliquid state. At a slightly higher temperature, the epoxy can thenprogress into a B-stage partially cross-linked cure which is quite hard,but very brittle. At this level the piezocomposite can be processedeasily by grinding to a thickness as a flat plate and applyingelectrodes. As a final step, the piezocomposite can then be reheatedslightly past the B-stage temperature where it softens considerably andcan be formed into shapes. Commonly, the piezocomposite is placed in awarm bottom half of a mold that allows the plate to take the shape ofthe mold, a top half of the mold is then clamped to the bottom half, andthen the mold is heated to a high final curing temperature. At thispoint, the epoxy filler cures to a fully cross-linked, tempered, stageover a period of time determined by the resin system. The resultingmolded piezocomposite now has a shape that takes on the shape of a lens.In this manner, a piezoelectrically active lens can be made that isquite rugged, eliminating the need for separate focusing lenses.

The next step provides an essential process step of the presentinvention. That is the molding of the transducer form to shape theelements and polymer (epoxy) combination into a shaped transducer (FIG.6 e). The form selected will dictate the focal range, depth and shape ofthe ultrasound focal zone. Thus the mold must be carefully determinedand created since it is not possible to electronically steer thetransducer when it is completed. When the transducer form is in themold, the mold and transducer form are heated until the resin/epoxy isheated past the B-stage of the filler material. This makes theepoxy/resin softer so the transducer form can be properly shaped. Thefinal shaped transducer (FIG. 6 f) is then coated with a conductivematerial and electronically connected (FIG. 6 g) to an activation devicesuch as a high voltage supply (not shown).

The reference numbers used in FIGS. 6 a-6 g are as follows:

-   -   10 a Piezoelectric ceramic    -   10 b Dized ceramic    -   10 c Filled with epoxy 13    -   10 d Back removed    -   10 e Heat molding    -   10 f planar transducer with irregularity    -   10 g Heat molded with irregularity and conductive layer 12 and        electrical connection 14

The transducer can be made to operate either narrowband (roughly at onefrequency) or broadband (over a range of frequencies). The decision onnarrow or broadband will be left to the designer to choose based on thespecific application. The designer should consider the beam pattern thevortex transducer will produce before finalizing the design. The beamshape will produce a double funnel focus field similar to that describedin the prior art; however, with the simplified transducer of the presentinvention, there will be no need for complex electronics to steer orfocus the beam, or run a complex pattern of individual elementactivations.

Additional modifications and variations of the present invention arepossible in light of the above teachings and the description providedherein is not to be meant as limiting the invention to the descriptionsprovided. It is understood those skilled in the art will be able toutilize the present teaching without substantial variation, and suchpractices are within the intended scope of the present invention and theappended claims.

1. A medical ultrasound transducer having an axis with a front and backsurface transverse to the axis, wherein an edge of the surface isaxially offset to produce at least one substantially annular 2 πm phaseshift focal region(s) when said transducer is excited, where m is anon-whole number value.
 2. A medical ultrasound transducer having anaxis with a front and back surface transverse to the axis, wherein twoor more edges of the surface are axially offset to produce a 2 πm phaseshift focal region for each offset when said transducer is excited,where m is any real number.
 3. The transducer of claim 2, wherein thetwo or more edges have the same axial offset value.
 4. The transducer ofclaim 2, wherein the two or more edges have different axial offsetvalues.
 5. The transducer of claim 1 where the transducer incorporates afiller material in front of the transducer or backing material in backof the transducer.
 6. The transducer of claim 1, being formed of asingle contiguous piezoelectric element.
 7. A method of creating amedical ultrasound transducer having a non-ideal focal region, themethod comprising the steps of: shaping a piezoelectric ceramic into adesired form, the form having a front end and a back end; dicing saidfront end create a plurality of elements, said elements being attachedto said back end and separated by dicing channels; filling said dicingchannels with an epoxy material and allowing said epoxy to gel; creatinga transducer form by removing said back end such that said elements areseparated from one another; pressing said transducer form into a moldand heating said transducer form such that the epoxy is heated above theB-stage and allowing the resin to cross link and cool in a set shape;treating at least one surface of the transducer form with a conductivematerial such that all elements are in contact with said conductivematerial; and making a shape irregularity in the transducer form suchthat the transducer will produce a non-ideal focal region when saidtransducer is excited.